Impulse noise in audio equipment, commonly referred to as "clicks" and "pops", can be caused by a number of factors, among them dust and scratches on the recording medium, static, and so forth. The removal of such noise has previously been a difficult problem due to its wide bandwidth, wide variation in amplitude, and the need to remove any such noise completely.
The prior art in this area has consisted primarily of devices which low pass filter, limit amplitude, limit slew rate, or blank the signal in the presence of impulse noise. Conventional audio equipment such as receivers and preamplifiers utilize low pass filters to limit the occurrence of impulse noise. This approach also limits the high frequency response of the system and at the same time only attenuates the impulse noise slightly. Automatic noise limiters, commonly used in vehicle radio equipment to remove ignition noise (a form of impulse noise) limit the signal excursion to a certain amplitude [1]. This method also only reduces the amplitude of the noise, in addition to greatly decreasing the dynamic range of a system in which it is used. FNT 1. Myers, R., "The Radio Amateur's Handbook," American Radio Relay League, Newington, CT, 1976.
Systems designed specifically for the elimination of impulse noise generally operate in one of two manners [2]. First, such systems often take advantage of the rapid change in amplitude caused by an impulse and supress it by limiting the slew rate of the audio circuitry. However, high amplitude, high frequency signals are adversely affected by such a slew rate limitation. FNT 2. Burwen, R. S., "A Dynamic Noise Filter," Journal of the Audio Engineering Society, Vol. 19, No. 2, February 1971.
The second approach to impulse noise limiting utilizes blanking circuits to eliminate the entire signal upon the detection of impulse noise. This approach does not adversely affect signal quality when no impulse noise is present; however, attempts to utilize the approach have not been effective in the complete elimination of an impulse and have not yielded the capability to handle wide variation in impulse duration.
The primary object of the present invention is to provide a critically improved time-delay system that is capable of separating impulse noise components, of both large and small size or energy, from the full range of audibly proper components of an audio signal. Generally, in combination with input retarding and output synchronizing circuits that enable detection and blanking of the undesired impulse noise components, the system comprises circuits, which: respond to the audio signal by producing a transformed signal having attenuated audibly proper components and intensified impulse noise components; respond to the transformed signal by producing an averaging signal characterized by instantaneous sizes that correspond to averaged sizes of predetermined time spans of the transformed signal; respond synchronously to the transformed signal and the averaging signal by producing a detection signal having specified components of sizes that vary functionally with sizes of the impulse noise components; and respond to the detection signal by producing a control signal having blanking components characterized by varying time spans that are functionally related to the sizes of the specified components.
Other objects of the present invention are to provide: filtration and differentation circuitry for generating the transformed signal; rectification circuitry for generating the averaging signal; comparison circuitry for generating the detection signal; and variable charge circuitry for generating the control signal.
A subsidiary object of the present invention is to control the slew rate during the blanking in the output synchronizing circuit in order to minimize introduction of noise as a consequence of operation of the system of the present invention.
In audio systems with more than one channel, the above circuitry is duplicated for each channel, although certain portions such as the impulse detection circuitry, the delay circuitry, and the blanking control circuitry may be shared between channels.